The present invention relates to an optical device in which a lens element is optically coupled to another optical element, more particularly to an optical device in which the alignment of the lens element and the other optical element is simplified.
Known alignment methods include passive methods such as the one disclosed by Tanaka et al. in IEICE Transactions on Electronics, Vol. E80-C, No. 1 (January 1997), pp. 107-111. This method aligns an optical fiber with a laser diode chip by mounting both of them on a supporting substrate referred to as a silicon platform. Photolithographic techniques are used to form a V-groove and fiducial marks simultaneously on the silicon platform. The optical fiber is mounted in the V-groove, the depth of which controls vertical alignment; the laser diode chip is mounted on a solder pad on the silicon platform and positioned in relation to the fiducial marks, which control horizontal alignment. Because of the high accuracy of photolithography, the laser diode chip can be positioned with sufficient accuracy to couple the emitted laser beam, which has a typical diameter of one to six micrometers (1-6 xcexcm), into the optical fiber within an alignment tolerance of one to two micrometers (1-2 xcexcm).
It would be desirable to extend this passive alignment technique to optical devices that also include lens elements, in particular to devices including diffractive lens elements such as computer-generated holograms (CGHs). Three-dimensional positioning of lens elements is more difficult, however. CGH elements cannot easily be mounted in V-grooves, for example. Accurate alignment of lens elements can be achieved by active techniques, in which the alignment is adjusted while test light is transmitted and the degree of optical coupling is measured, but active alignment has the disadvantages of taking time and requiring expensive test equipment.
An object of the present invention is to simplify the alignment of a lens element, such as a CGH element, with an optically coupled element mounted on a supporting substrate.
The invented optical device has a supporting substrate, an optical element mounted on the supporting substrate, an optical substrate, and a lens element formed in the optical substrate. The supporting substrate includes at least an etching stopper layer and a surface layer. The surface layer is partially removed by etching to create a window in which the etching stopper layer is exposed. Alternatively, the surface layer and the etching stopper layer are both partially removed to create a window in which a supporting layer underlying the etching stopper layer is exposed. The optical substrate is mounted on the surface of the exposed layer. The optical element is mounted on the surface layer.
In the invented optical device, accurate alignment can be achieved by passive two-dimensional horizontal positioning techniques, alignment in the third dimension being controlled automatically by the thickness of the surface layer, or the combined thickness of the surface layer and etching stopper layer, or this thickness and the shape and depth of a V-groove. The alignment process is accordingly quick and easy.
A silicon-on-insulator substrate of the type used for semiconductor integrated circuits can be used as the supporting substrate. Such substrates typically have a thin and very flat buried oxide (BOX) layer that can function as the etching stopper layer. The exposed surface in the window then has a high degree of flatness.
The optical element and optical substrate can be positioned in relation to marks in the surface layer. Such marks can be formed with high precision by photolithography and etching, leading to very accurate alignment of the optical element and the lens element.
The optical substrate can also be positioned in relation to the sides of the window, which can be defined accurately by photolithography.
The thickness of the surface layer, or the combined thickness of the surface layer and the etching stopper layer, should be accurate to within the alignment tolerance of the optical element and the lens element. A typical requirement is a surface layer one hundred micrometers thick with a thickness tolerance of plus-or-minus half a micrometer.
The lens element may be a CGH element formed in a side surface of the optical substrate, this surface being oriented perpendicular to the bottom surface of the optical substrate, which rests on the surface exposed in the window. Alternatively, the lens element may be another type of diffractive lens element, or a refractive lens element, including various types of bulk lens elements, and including aspherical lens elements.
The optical element mounted on the surface of the supporting substrate may be, for example, an optical fiber, a laser diode, or a photodiode. The optical device may have a plurality of such optical elements mounted on the same supporting substrate, and a plurality of lens elements formed in the same optical substrate.